Many computer paint and drawing programs allow a user to fill an area with a single and/or multiple colors. In the case of multiple colors, computer paint programs can automatically blend between the colors specified. This smooth blending between colors produces a color gradient. Conventional types of color gradients include linear gradients, radial gradients, and gradient meshes.
To fill a region with a linear gradient, a line segment is specified (e.g., on the drawing canvas) and colors are associated with points along the line segment. For example, one endpoint of the line segment can be mapped to blue, the other endpoint can be mapped to red, and an intermediate point on the line segment can be mapped to yellow. The color at any other point on the line segment is determined by interpolating (e.g., using linear or cubic interpolation) between the specified colors. At any other point in the region to be filled, the color is defined to be the same as the color of the nearest point on the line segment. The user can choose multiple colors along the line segment and can specify the technique used to interpolate between colors, but a linear gradient is defined by a single line segment. The linear gradient is the most basic gradient of all and is typically not complex enough for advanced effects.
Radial gradients can be used to create slightly more complex effects than linear gradients. To create a radial gradient, a line segment is specified by selecting an initial point and an end point. As with linear gradients, colors are associated with points along the line segment, and the color of any intermediate point on the line segment is determined by interpolating between the specified colors. However, unlike linear gradients, the color of any other point in the region to be filled is determined according to its distance from the initial point of the line segment. Thus, the resulting radial gradient comprises generally concentric rings of color that vary (e.g., smoothly and continuously). Along the initial line segment, colors blend smoothly as they do in the case of linear gradients. Points on other rays that begin at the initial point are colored in the same way, according to their distance from the initial point.
A gradient mesh is created using a two dimensional Bézier surface, specified by a mesh of control points, each of which is associated with a color. Each point in the region to be filled is colored by interpolating between the colors of nearby control points, using standard Bézier surface evaluation. The mesh gradient is more advanced than both linear and radial gradients; however, complex mesh gradients put a heavy burden on the users, as they can only be achieved by specifying the positions and colors of many control points.
Thus, linear and radial gradients are easy to use and to specify, but they typically generate simple color gradients. A gradient mesh is more powerful, but is more difficult for designers to use. For example, in order to imitate the smooth gradation of color produced by an airbrush, the user is forced to think about increasing the complexity of the control mesh, rather than simply drawing a color curve. It is also difficult to define a mesh gradient to fill a non-rectangular region, since Bézier surfaces are typically defined using quadrilateral meshes. Achieving a desired appearance is also time-consuming and non-intuitive, since more complicated effects generally require the user to individually move and color many control points.